Trapped Radial Oscillations of Gaseous Disks around a Black Hole

Abstract In gaseous disks around a black hole, the epicyclic frequency does not increase monotonically inward in the radial direction. The effects of general relativity make it the maximum at 4a, decrease it inward and eventually make it zero at 3a, where a is the Schwarzschild radius. This implies that a low-frequency wave excited in the inner disk is trapped and cannot propagate outward much beyond the region of radius 4a. Characteristics of this trapped oscillation are examined under some approximations. For typical values of parameters, the oscillation period of 100 d, which is a typical observed period of time variations of QSOs and Seyfert galaxies, is realized when the mass of the black hole is 109–1010M◉.

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